1. Field of Invention
The present invention relates to a planar panel display and a manufacturing method thereof. More particularly, the present invention relates to an interferometric modulation pixel and a manufacturing method thereof.
2. Description of Related Art
Planar displays are extremely popular in the portable and limited-space display market because they are lightweight and small. To date, in addition to liquid crystal display (LCD), organic light-emitting diode (OLED) and plasma display panel (PDP) display panels, a module of the optical interference display has been investigated.
The features of an interferometric modulation pixel of the optical interference display include low electrical power consumption, short response time and bi-stable status. Therefore, the optical interference display can be applied in planar display panels, especially in portable products such as mobile phones, personal digital assistants (PDA), and portable computers.
U.S. Pat. No. 5,835,255 discloses a modulator array for visible light, and an interferometric modulation pixel of the modulator array can be used in a planar display panel. FIG. 1A illustrates a cross-sectional diagram showing an interferometric modulation pixel in the prior art. Every interferometric modulation pixel 100 comprises a bottom electrode 102 and a top electrode 104. The bottom electrode 102 and the top electrode 104 are separated by supports 106, thus forming a cavity 108. The distance between the bottom electrode 102 and the top electrode 104, that is, the depth of cavity 108, is D and is usually less than 1 μm. The bottom electrode 102 is a light-incident electrode and partially absorbs visible light according to absorption rates of various wavelengths. The top electrode 104 is a light-reflection electrode which is flexed when a voltage is applied to it.
A white light is usually used as an incident light source for the interferometric modulation pixel 100 and represents a mixture of various wavelengths (represented by λ) of light in the visible light spectrum. When the incident light shines through the bottom electrode 102 and enters the cavity 108, only the visible light with wavelength (λ1) corresponding to the formula 1.1 is reflected back, that is,2D=Nλ1  (1.1),wherein N is a natural number.
When twice the cavity depth, 2D, equals one certain wavelength λ1 of the incident light multiplied by any natural number, N, a constructive interference is produced, and a light with the wavelength λ1 is reflected back. Thus, an observer viewing the panel from the direction of the incident light will observe light with the certain wavelength λ1 reflected back at him. The display unit 100 here is in an “open” state, i.e. a “bright” state.
FIG. 1B illustrates a cross-sectional diagram of the interferometric modulation pixel 100 in FIG. 1A after a voltage is applied to it. Under the applied voltage, the top electrode 104 is flexed by electrostatic attraction toward the bottom electrode 102. At this moment, the distance between the walls 102 and 104, the depth of cavity 108, becomes d and may equal to zero. The D in the formula 1.1 is hence replaced with d, and only the visible light with another certain wavelength λ2 satisfying the formula 1.1 produces constructive interference and reflects through the top electrode 102. However, in the interferometric modulation pixel 100, the bottom electrode 102 is designed to have a high absorption rate for the light with the wavelength λ2. Thus, the incident visible light with the wavelength λ2 is absorbed, and the light with other wavelengths is annulled by destructive interference. The incident visible light of all wavelengths is thereby filtered, and the observer is unable to see any reflected visible light when the top electrode 104 is flexed. The interferometric modulation pixel 100 is now in a “closed” state, i.e. a “dark” state.
As described above, under the applied voltage, the top electrode 104 is flexed by electrostatic attraction toward the bottom electrode 102 such that the interferometric modulation pixel 100 is switched from the “open” state to the “closed” state. When the interferometric modulation pixel 100 is switched from the “closed” state to the “open” state, the voltage for flexing the top electrode 104 is removed and the top electrode 104 elastically returns to the original state, i.e. the “open” state as illustrated in FIG. 1A.
In light of foregoing, the interferometric modulation pixel 100 is obtained by combining thin film interference principles of optics with reflective plate and microelectromechanical system (MEMS) processes. In a MEMS process, the cavity 108 is formed by etching a sacrificial layer between the bottom electrode 102 and the top electrode 104. The material used as the sacrificial layer is usually metal, polysilicon or amorphous silicon. The silicon-based material is inexpensive compared to the metallic material and is often preferred when developing manufacturing processes. However, if the etchant that is used to remove the sacrificial layer does not properly etch selectively, the surface of the bottom electrode 102 is damaged such that the cavity depth D and the optical thin film of the bottom electrode are adversely affected. That is, the reflected wavelength λ1 is different from what is intended, damaging the color uniformity of the optical interference display.